2D conjugated microporous polyacetylenes synthesized via halogen-bond-assisted radical solid-phase polymerization for high-performance metal-ion absorbents

The paper reports the first free-radical solid-phase polymerization (SPP) of acetylenes. Acetylene monomers were co-crystalized using halogen bonding, and the obtained cocrystals were polymerized. Notably, because of the alignment of acetylene monomers in the cocrystals, the adjacent C≡C groups were close enough to undergo radical polymerization effectively, enabling the radically low-reactive acetylene monomers to generate high-molecular-weight polyacetylenes that are unattainable in solution-phase radical polymerizations. Furthermore, the SPP of a crosslinkable diacetylene monomer yielded networked two-dimensional conjugated microporous polymers (2D CMPs), where 2D porous polyacetylene nanosheets were cumulated in layer-by-layer manners. Because of the porous structures, the obtained 2D CMPs worked as highly efficient and selective adsorbents of lithium (Li+) and boronium (B3+) ions, adsorbing up to 312 mg of Li+ (31.2 wt%) and 196 mg of B3+ (19.6 wt%) per 1 g of CMP. This Li+ adsorption capacity is the highest ever record in the area of Li+ adsorption.


Measurement.
The proton nuclear magnetic resonance ( 1 H NMR) spectra were recorded at room temperature on a Bruker (Germany) BBFO400 spectrometer (400 MHz) and AV400 spectrometer (400 MHz).
DMSO-d6 was used as the NMR solvent. The residual non-deuterated solvents and tetramethylsilane (TMS) were used as the internal standards for 1 H NMR analysis (calibration of chemical shift).
The Fourier-transform infrared spectroscopy (FTIR) analysis was carried out on a Bruker ALPHA FTIR (Bruker, US) spectrometer. KBr was used as a matrix for FTIR.
The scanning electron microscopy (SEM) images were obtained with a JSM-7600F Schottky field emission scanning electron microscope (JOEL, Japan) operated at 5 kV.
The cocrystal monomers were moulded using 2T Mini-Pellet Press (Specac, UK) to form sheets with a diameter of 7 mm. The polymers for the resistivity measurements were pressed into thin films with a diameter of 13 mm using 15T Manual Hydraulic Press (Specac).
The atomic force microscopy (AFM) images were obtained with a MultiMode Scanning Probe Microscope (Bruker) in the ScanAsystTM mode using a cantilever (ScanAsyst-Air, Bruker).
The transmission electron microscopy (TEM) images were obtained with a JEOL (Tokyo, Japan) TEM-1400 transmission electron microscope operated at 100 kV. The TEM grid was carbon- Single crystal X-ray diffraction frames were analysed with a Bruker D8 QUEST (Bruker) and integrated with the Bruker SAINT software package using a narrow-frame algorithm. The data were corrected for absorption effects using the Multi-Scan method (SADABS). The structures were solved by XT VERSION 2014/5 and refined by SHELXL-2017/1 (Sheldrick, 2017 programs, respectively. The refinement was carried out by full-matrix least-squares on F 2 . Hydrogen atoms were placed using standard geometric models and with their thermal parameters riding on those of their parent atoms. The powder X-ray diffraction (PXRD) analysis was carried out on a Bruker D8 ADVANCE (Bruker) from 10.000° to 79.994° (step size 0.020) using CuKa radiation (wavelength (l) = 1.541874 Å). The parameters (2 theta (2q) and full width of half maximum (FWHM) shown in Supplementary Table 3) were calculated from PXRD spectra and data calculated from Origin software. The crystallite size was calculated using the modified Scherrer equation (least square). [1] The layers distance/plane spacing (dhkl), microstrain (e), crystallite size (D), and dislocation density (d) were calculated from the following formula: where: b is the radians of FWHM; q is the incident angle (the angle between the incident ray and the scatter plane) [°]; K is the Scherrer constant, which is a dimensionless shape factor (K = 0.9); l is the radiation wavelength (l = 0.1541874 nm); D is the crystallite size [nm] in the powder sample and was obtained from the intercept of the plot of lnb vs ln(1/cosq) according to equation (1) (or (2)); n is an integer (n = 1).
The surface resistivity (ρs (Ω/sq)) values of the PPDA-CMP-1 and PPDA synthesized in solution polymerization were determined by a four-point technique with a Loresta-GP resistivity meter (Mitsubishi Chemical Analytech (Japan), MCP-T610) at room temperature. The polymers are pressed to form thin films using a manual hydraulic press (15T) prior to the analyses. The Loresta-GP MCP-T610 meter included a standard accessories PSP probe (MCP-TP06P, 4-pins, inter-pin distance 1.5 mm, pin points 0.26R, spring pressure 70 g/pin) and a probe checker (MPC-TRPS).
The thicknesses (L (cm)) of the polymer films were measured by a high-precision digital caliper (Fowler ProMax-Cal, Japan). The electrical conductivity (σ (S cm -1 )) values were calculated according to equation (6): The surface areas of PPDA-CMP-1, PPDA-CMP-2, and PPDA-CMP-3 were analyzed with a Micromeritics 3FLEX (Micromeritics, USA) analyzer at -196 o C. Before measurement, the samples were degassed totally in the nitrogen (N2) atmosphere at 120 o C for 24 h and then backfilled with N2. We studied the relative pressure (P/P0) from 0 to 1 at -196 o C, where P0 is the saturated pressure of adsorbent (N2). The specific surface areas (m 2 g -1 ) were determined via Brunauer-Emmett-Teller (BET) model at the linearized P/P0 range from 0.06 to 0.14 according to equation (7): where Q is the volume of nitrogen gas adsorbed per weight of adsorbent (cm 3 g -1 STP) at a given relative pressure (P/P0), Qm is the volume of nitrogen gas adsorbed to form the monolayer per weight of adsorbent (cm 3 g -1 STP), and cBET is the BET constant (STP is standard condition at temperature 273 K and pressure 1 atm). The plot of 1/(Q((P0/P)-1)) vs P/P0 was linear for all studied cases ( Supplementary Fig. 37), which indicates the formation of the monolayer in this range (P/P0 = 0.06-0.14). From the slope and intercept of the plot, the cBET and Qm values were determined. The BET specific surface area (SBET, cm 2 g -1 ) was calculated from Qm according to equation (8): where the NA is Avogadro's number (6.022×10 23 mol -1 ), Am is the molecular cross-sectional area for liquid N2 (0.162 nm 2 ), and Mv is the molar volume for the ideal gas at STP (22414 cm 3 mol -1 ).
The average pore diameter (dBET (nm)) was calculated at P/Po = 0.99, assuming a cylindrical pore. At P/Po = 0.99, the pores were assumed to be completely filled with N2 and the total volume (VBET) of the adsorbed N2 can be considered as the total pore volume. The dBET (nm) was calculated according to equation (9): where VBET is the Q at P/Po = 0.99.
An inductively coupled plasma optical emission spectrometer (ICP-OES) (ICAP 6500, Thermo Scientific, US) was used to determine the metal ion concentrations (ppm or mg L −1 ) of the samples.
The simultaneous axial and radial view of the plasma was enabled by a synchronous vertical dual view (SVDV). The analytical conditions are: radio frequency (RF) power 1150 W, nebulizer gas flow 0.08 L min −1 , auxiliary gas flow 1.0 L min −1 , plasma gas flow 12.0 L min −1 , and signal accusation time 3 s/replicate for 3 replicates. All standards and samples were dissolved (diluted) in nitric acid (2 wt% in ultrapure water) before analysis. Metal ions Li, Rb, and B were analysed individually at wavelength l = 670.784, 780.023, and 249.773 nm, respectively, with the correlation factor of the calibration curve R 2 > 0.999. For all the elements determined, the uncertainty of the analytical concentration (RSD) was <5%.
Dichloromethane would initially evaporate relatively quickly. Once the solution has been saturated, cocrystals began to form. Because the solution was not pure dichloromethane but a mixture of monomer, linker, photo-initiator, and solvent, the evaporation gradually slowed down and needed a reduced pressure to completely remove the solvent. The low rotation speed (10 rpm) would prevent vibrations to the solution and was applied not to disturb the growth of the cocrystals.

Monomer Alignments and Four Possible Monomer Addition (Propagation) Patterns.
Supplementary Table 1 shows four possible monomer addition (propagation) patterns, i.e., path A (parallel alignment + head-to-head and tail-to-tail propagation), path B (parallel alignment + head-to-tail propagation), path C (zigzag alignment + head-to-head and tail-to-tail propagation), and path D (zigzag alignment + head-to-tail propagation). Supplementary Table 1 shows singlecrystal X-ray crystallography data. The p-p distance between two linkers, hence the distance of two pyridyl (R) groups of monomers, was 3.571-7.391 Å. The propagation in the parallel alignment (paths A and B) will give R-R distances of 1.54-2.49 Å in the generated polymers, which are much shorter than the original R-R distances (3.571-7.391 Å) (hence p-p distances) in the monomer cocrystals and will cause significant deformation of the crystal structures. Therefore, paths A and B might occur but would not be major paths in the present polymerizations. The propagation in the zigzag alignment (paths C and D) will give parallel (every other) R-R distances of 4.42-4.98 Å in the generated polymers, which are close to the original parallel R-R distances (3.571-7.391 Å) (hence p-p distances) in the monomer cocrystals and will suppress the deformation of the crystal structures. Therefore, paths C and D would be more likely to occur than paths A and B. Electronically and sterically, path D (head-to-tail propagation) would be more favorable than path C (head-to-head and tail-to-tail propagation). This is because, in path D, the electron-rich and sterically hindered propagating radical carbon (with an electro-donating R group) can react with an acetylene monomer at the electron-deficient and sterically less hindered tail carbon (C-H) rather than the electron-rich and sterically more hindered head carbon (C-R). Thus, path D would be favorable. However, other paths (A-C) might also occur to some extents.
For the di-acetylene monomer (1), path D can occur in two ways because two acetylenes are present in one monomer ( Supplementary Fig. 1). In one way, monomers are linked in a face-toface manner, where two monomers are bridged via two bonds, forming an intra-ladder (single ladder) polymer structure (structure D1 in Supplementary Fig. 1). In another way, monomers are linked in a staggered manner, where one monomer is linked with one monomer via one bond and another monomer via another bond, forming an inter-ladder polymer nanosheet structure (structure D2 in Supplementary Fig. 1). These two ways might also operate in mixed manners, forming mixed intra-inter-ladder polymer nanosheet structures; an example is structure D3 in Supplementary Fig.   1. Experimentally, we observed polymer nanosheets (as described in the manuscript), and hence the structures would not be a pure form of structure D1 but be structure D2 and mixed structures exemplified by structure D3. We put structure D2 in Fig. 1d as a guide. It should be noted that other structures might also be formed. Different propagation patterns (paths A-D) and formation of mixed structures exemplified by structure D3 might occur simultaneously. Also, there might be defects in the monomer cocrystal structures. Thus, structure D2 is viewed as one of the probable structures ( Supplementary Fig. 1).
In our previous solid phase polymerization (SPP) of vinyl monomers, [2] tacticity (stereo structure) was not regulated because the terminal C-C • bond can rotate even in the limited freedom in the cocrystal. Meanwhile, monomer addition patterns (head/tail configuration) would mostly be determined by the alignment of monomers, because monomer re-alignment (entire molecular rotation) in the cocrystals would hardly occur due to the limited freedom. Polyacetylenes consist of sp 2 carbons in the backbones and hence are rigid. Therefore, mobility of the chain end radical is restricted, which would also assist the retention of monomer alignment structures in the polymer structures.

UV Irradiation to Two-Component Cocrystal Monomers.
The two-component cocrystal monomers of monomers 1−5 and XB linkers 6−8 were prepared with vaporization method as described (without photo-initiator DMPA). The obtained cocrystal monomers were put in a vial. The vial was capped with a rubber septum, and oxygen was removed with an argon flow for 10 min. The cocrystal monomers were then irradiated with UV light (l = 365 nm) at room temperature for 40 h. The irradiated samples were then analysed with 1 H NMR  and GPC, showing no formation of polymers for all studied cases.

Photo-SPP of Three-Component Cocrystal Monomers. [2]
The cocrystal solid powder obtained above was moulded using a 2T mini-hand hydraulic press to form a round-shape sheet with a diameter of 7 mm. The sheet was put in a vial. The vial was capped with a rubber septum, and oxygen was removed with an argon flow for 10 min. The sheet was irradiated with UV light (l = 365 nm) at room temperature for 40 h. DMPA gradually decomposed to continuously supply radicals under 365 nm UV LED during the polymerization rather than to generate radicals in a bursting manner. In the present study, DMPA was sufficient to attain nearly quantitative monomer conversions. (We also studied the SPP of 1·6 at varied polymerization time (0.25-40 h) and with varied amounts of photo-initiator (0.01-0.67 equiv to linker and 0.005-0.335 equiv to the monomer) (Supplementary Table 2). The polymers formed at shorter times (0.25, 3, 7, and 24 h) and smaller amounts of photo-initiator (0.01 and 0.10 equiv to linker (0.005 and 0.05 equiv to monomer, respectively)) did not reach high monomer conversions or retain the grain (cocrystal) structures, suggesting that the SPP for 40 h with 0.67 equiv of DMPA to linker (0.335 equiv to monomer) is an optimal SPP condition.) The sheet was washed with ethanol (20 mL) three times to remove soluble polymer, residual monomer (if present), XB linker, and residual DMPA, yielding purified insoluble polymer. The soluble part of the polymer in ethanol (10 mL) was reprecipitated into hexane (100 mL) to remove the residual monomer, XB linker, and DMPA, and analysed with GPC to determine the molecular weight and dispersity of the soluble part of polymer. The monomer conversion was determined by analyzing the ethanol solution (containing soluble polymer and monomer) using 1 H NMR and the weight of insoluble polymer (monomer conversion = (total amount of soluble polymer and insoluble polymer)/(total amount of monomer, soluble polymer, and insoluble polymer)). The monomer cocrystals 1·6, 1·7, and 1·8 and their polymers obtained via SPP were analysed with TEM. The dried solid samples were grinded into fine powders and directly attached onto the Cu grids (Supplementary Figs. 23-25).

Removal of Linker After Polymerization.
After the SPP, the polymerized 1·6, 1·7, 1·8, 2·6, and 2·7 cocrystals were purified (washed) using ethanol to remove the linkers. The FTIR analysis of the washed polymers showed no C-F and C-I peaks of the linkers, demonstrating complete removal of the linkers from the polymers .

Solution-Phase Polymerization (Comparison Experiment).
In a typical run, monomer 1 (0.10 g, 0.79 mmol) and DMPA (67.0 mg, 0.26 mmol) were dissolved in dichloromethane in a vial, in which XB linker was not added. The solution formed a thin liquid layer at the bottom of the vial. The vial was capped with a rubber septum, and oxygen was removed with an argon flow for 10 min. The solution was irradiated with UV light (l = 365 nm) at room temperature for 40 h, yielding polymer. The solution was dropped in ethanol (10 mL) to precipitate polymer (insoluble polymer in ethanol) for separating out from soluble polymer, residual monomer, and residual DMPA. The insoluble polymer was separated by centrifugation.
The soluble part of the polymer in ethanol (10 mL) was reprecipitated into hexane (100 mL) to remove the residual monomer and DMPA, and analysed with GPC to determine the molecular weight and dispersity of the soluble part of the polymer. The monomer conversion was determined by analyzing the ethanol solution (containing soluble polymer and monomer) using 1 H NMR and the weight of insoluble polymer (monomer conversion = (total amount of soluble polymer and insoluble polymer)/(total amount of monomer, soluble polymer, and insoluble polymer)).

Doping PPDA With Iodine (I2) Vapor. [3]
In a typical run, 18 g iodine powder was loaded into a 20 mL capped glass vial and allowed for reaching solid-vapor equilibration of the iodine inside the vial at 100 °C for 10 min. In parallel, the PPDA-CMP-1 polymer film was heated in a separate vial at 100 °C for 10 min. The polymer film was subsequently placed into the iodine vial, which was capped tightly and heated at 100 °C for 30 mins. The film of the PPDA obtained in the solution-phase polymerization was studied similarly.

Exfoliation of PPDA-CMPs.
PPDA-CMP-1 (4 mg) was dispersed in 4 mL of g-butyrolactone (GBL) and sonicated for 30 mins to obtain a 0.1 wt% dispersed solution. A part of the solution was further diluted 500 times in GBL to obtain a 2´10 −4 wt% dispersed solution. The two solutions (0.1 and 2´10 −4 wt% solutions) were heated at 50 °C for 5 days with gentle stirring to induce exfoliation.
The ion-adsorbed PPDA-CMPs were rinsed with water three times to fully remove ions possibly covering the surface of the CMP powder and dried under vacuum for 24 h to obtain the ionabsorbed PPDA-CMP-1, PPDA-CMP-2, and PPDA-CMP-3. The solution parts before and after adsorption were filtered, diluted 500 times (with HNO3 2 wt% in ultrapure water), and analysed with ICP-OES to determine the ion concentrations in the solutions before and after adsorption (Tables 2 and 3 and Supplementary Tables 6 and 7). The content of metal ion (wt%) was calculated according to equation (10): where: C0 is the concentration of metal ion before adsorption [ppm or mg L −1 ]; C is the concentration of metal ion after adsorption [ppm or mg L −1 ]; Vsol is the volume of the metal ion solution [L]; The rinsed water was also analysed using ICP-OES, showing the amount of the ions covering the CMP surface was negligible (below the analytical detection limit) compared with the amount of ions adsorbed inside the CMP in all cases.  Table 1, entry 1). (a) Paths A (orange arrows) and B (green arrows) in parallel alignments with monomer distances of 3.535−4.813 Å (tail-to-tail) and 3.648−4.903 Å (head-to-head) (path A) and 3.393−4.232 Å (head-to-tail) (path B). (b) Paths C (pale blue arrows) and D (pink and dark blue arrows) in zigzag alignments with monomer distances of 3.663−3.724 Å (tail-to-tail) and 4.066−4.097 Å (head-to-head) (path C), and 3.584-7.002 Å (head-to-tail) (path D). The π-π distance between two linkers was 3.885-4.321 Å. The molecular packing views down crystallographic b axis in ball-and-stick and space-filling representation are constructed using the crystallographic information file and software package Mercury 3.10.3. Carbon: gray; hydrogen: white; fluorine: yellow; nitrogen: blue; iodine: magenta. Halogen bonds are presented as light blue dotted lines.   Table 1, entry 2). (a) Paths A (orange arrows) and B (green arrows) in parallel alignments with monomer distances of 4.349 Å (head-to-head and tail-to-tail) (path A), and 4.068 Å (head-to-tail) (path B). (b) Paths C (blue arrows) and D (pink arrows) in zigzag alignments with monomer distances of 4.616 Å (head-tohead) and 5.780 Å (tail-to-tail) (path C), and 5.121−5.428 Å (head-to-tail) (path D). The π-π distance between two linkers was 4.187 Å.  Table 1, entry 2). (a) Paths A (orange arrows) and B (green arrows) in parallel alignments with monomer distances of 4.349 Å (head-to-head and tail-to-tail) and 4.068 Å (head-to-tail), respectively. (b) Paths C (blue arrows) and D (pink arrows) in zigzag alignments with monomer distances of 4.616 Å (head-to-head) − 5.780 Å (tail-to-tail) (path C) and 5.121 Å ((head-to-tail (shorter)) − 5.428 Å (head-to-tail (longer)) (path D). The π-π distance between two linkers was 4.187 Å.  Table 1

(C 6 F 3 I 3 , blue), polymer from 1·7 before linker removal (orange), and polymer (PPDA-CMP-2) after linker removal (washing with ethanol) (green) (KBr).
The =C-H stretch, non-aromatic C=C bonds (polyacetylene backbone), and aromatic C=C bonds (pyridine) appeared at 3047, 1721, and 1639 cm -1 , respectively. The stretch at 1036 cm -1 and stretch at 649 cm -1 for XB linker 7 disappeared after washing with ethanol, indicating complete removal of XB linker 7.   The polymer structure is not an actual experimental X-ray structure but is expected from the monomer cocrystal structure.        a π-π distance between two linkers (average π-π distance between two linkers for entry 6). b hh = head-to-head, tt = tail-to-tail, and ht = head-to-tail. c R% is the R-factor in refinement using Bruker SHELXTL software, showing the discrepancy index between the experimental X-ray diffraction data and crystallographic model. Normally, R% less than 10% indicates a good fit.   Table 1, entry 1. c Amounts of DMPA at 0.67 equiv to linker and 0.335 equiv to monomer (entry 1), 0.01 to linker and 0.005 equiv to monomer (entry 2), and 0.10 equiv to linker and 0.05 equiv (entry 3) to monomer. a Rp is the R profile factor in Rietveld refinement, showing the discrepancy index between the experimental and calculated spectra. Normally, Rp less than 10% indicates a good fit. b Match ratio (%) = (the number of peaks of the polymer identically matched with those of the monomer (red))/ S{(the number of peaks of the polymer identically matched with those of the monomer) + (the number of shifted peaks in the polymer (blue)) + (the number of new peaks appeared in the polymer (green, if applicable)) + (the number of peaks present in the monomer but disappeared in the polymer (green, if applicable))} ´ 100%. The matching ratio was calculated in pairs (entries 1 vs 2, 3 vs 4, 5 vs 6, 7 vs 8, 9 vs 10, and 11 vs 12). 2 Solution PPDA NA (< 10 -9 ) a 8.3 × 10 -5 a Below the detection limit of the utilized instrument.  Fig. 37).  All the X-ray intensity data were measured at the temperature 100 (±2) K and wavelength λ = 0.71073 Å, with Multi-Scan absorption correction.